1. Field of the Invention
The present invention relates to a semiconductor laser chip, a semiconductor laser device, and a semiconductor laser chip manufacturing method.
2. Description of Related Art
Nitride semiconductors which are compounds of a group III element such as Al, Ga, or In and a group V element N are promising semiconductor materials for light emitting elements, power elements, and the like because of their band structure and chemical stability. Various applications of nitride semiconductors have therefore been tried, and one of those applications is a nitride semiconductor laser element, which is used as the light source of an optical information recording device such as an optical disk drive. Nitride semiconductor laser elements have achieved reliability and cost reduction in recent years, owing to the use of GaN substrates, an advance in crystal growth technology, well-designed element structures, improved wafer processing technology, and other factors, and have created a market as commercial products.
The expectation on nitride semiconductors as a fluorescent material excitation light source is also high because of their short oscillation wavelength. A typical application to a fluorescent material excitation light source is a white LED using a nitride semiconductor. In recent years, high power lasers made from nitride semiconductors, too, are attracting attention for uses in the next-generation directional lights, television sets, and the like where directionality and high power are required. In these uses, semiconductor lasers generate a large amount of heat and how to dissipate heat is important.
A known way to improve heat dissipation performance is to connect a plurality of wires to a p-side electrode (positive electrode) of a nitride semiconductor laser element. An example of this method is described in JP 3618989 B.
FIG. 40 is a perspective view of a conventional nitride semiconductor laser device described in JP 3618989 B. As illustrated in FIG. 40, the nitride semiconductor laser device described in JP 3618989 B includes a semiconductor laser element 1000, an electrode terminal 1100, and a plurality of wires 1200. The semiconductor laser element 1000 has a substrate 1010 on which a laminated structure 1020 is formed from a nitride semiconductor necessary for laser oscillation. In the laminated structure 1020, a stripe-like (strip-like) oscillation region (optical waveguide) 1030 is formed. A p-side ohmic metal contact 1040 is formed on a top surface of the laminated structure 1020. The plurality of wires 1200 are connected to a top surface of the p-side ohmic metal contact 1040 in a manner that distributes the wires 1200 along the length of the oscillation region 1030. In the thus structured nitride semiconductor laser device of JP 3618989 B, heat generated in the oscillation region 1030 is dissipated via the plurality of wires 1200 connected to the p-side ohmic metal contact 1040.
Semiconductor lasers for uses in the next-generation lights, television sets, and the like, or industrial lasers for processing uses, generate a large amount of heat, which means that their laser elements deteriorate fast. It is therefore a common practice to make laser elements for these uses a “broad area type,” where the ridge stripe (ridge width) is set wide.
The inventors of the subject application conducted a reliability test on a broad area nitride semiconductor laser element having a ridge width of 7 μm and employing the heat dissipation measure of JP 3618989 B. It was found as a result that the element life span was not improved significantly by the conventional heat dissipation method described above alone.
While the light source of an optical information recording device is generally required to have a life span of about several thousand hours to ten thousand hours, the requested life span of an excitation light source is far longer at several ten thousand hours to hundred thousand hours. The optical power necessary for use as an excitation light source is very high at one watt to several watts, and the amount of heat generated by a laser element for use as an excitation light source is accordingly several times larger than the amount of heat generated by a laser element for use as the light source of an optical information recording device. Considering these facts, reasons for the poor result of the reliability test are presumably because slowing down the deterioration of a light emitting layer by setting the ridge width wide and thus lowering the optical density does not produce a sufficient effect, and because only a limited amount of heat dissipates via the wires.
In short, the conventional heat dissipation method described in JP 3618989 B is not satisfactorily effective in enhancing heat dissipation performance and still leaves room for improvement in heat dissipation performance.
The conventional method which connects a plurality of wires to the p-side electrode thus has a problem of difficulties in improving heat dissipation performance satisfactorily. The insufficient heat dissipation performance causes another problem by degrading the element characteristics and lowering reliability. The problems are serious particularly when a laser element is used as an excitation light source and a large amount of heat is generated.